Cesium Primary Frequency References

Abstract
Primary frequency standards with stated inaccuracies of δf/f∼10-15 or slightly better are in use today in several national timing laboratories. These standards, which are the most accurate in the world today, use laser-cooled cesium atoms to obtain this level of performance. We discuss the operation of these standards as well as possible future improvements that should see the inaccuracy of the realization of the second fall to the 10-16 level over the next 5–10 years.